From logs and my own expiriece few years ago.

Affected by fear poison:
1) You can't engage your foe normally: cast icicle <name>, murder <name> will cause "You are too much in fear to do so".
Still can engage with area spell. (not sure)
2) You flee immediately when got single hit in battle, if not lagged (by overs or your own commands)
3) Rarely you begin to move random directions (outside of battle) "overhelmed by fear" and yell something in panic.